1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to controlling a velocity of a motor, and more particularly, to controlling a velocity of a motor by calculating an accurate reference estimation velocity of the motor in an overall velocity period of the motor.
2. Description of the Related Art
Generally, a vector control method for calculating an accurate rotation velocity of a motor is broadly used in industry fields requiring high performance of the motor. In the vector control method, a position sensor having a high resolution, such as an optical encoder and a resolver, is used because the vector control method requires continuously sensing a position of a rotor. However, the position sensor having a high resolution is expensive, and restricts an installation environment of an apparatus for controlling the motor.
A sensorless vector control method by which a position of the rotor is estimated without the sensor is proposed for solving the defect caused by using the position sensor.
In the sensorless vector control method, a phase excited for obtaining a maximum torque while the motor rotates is determined according to the position of the rotor. Also, the position of the rotor is measured by an inverse electromotive force while the rotor rotates. On the basis of the above, a proper phase for obtaining a maximum torque is selectively excited so that the rotation of the motor is controlled.
However, a problem arises in that it is difficult or impossible to estimate the rotation velocity of the motor in a zero-speed or a low-speed period of the motor because the rotation velocity of the motor is estimated by the inverse electromotive force or by estimating a magnetic flux of the rotor.
A sensorless control method and apparatus of a permanent magnet synchronous motor for continually controlling the velocity in a overall velocity period of the motor is disclosed in U.S. Pat. No. 6,081,093.
The sensorless control method and the apparatus disclosed in U.S. Pat. No. 6,081,093 modifies a command velocity and an estimation velocity with a weight using a membership function, and sums up the command velocity and the estimation velocity modified with the weight for smooth conversion from a low speed to a high speed. That is, a relative importance of the weight of the command velocity is larger in an initial start period or a low-speed period.
However, if the sensorless control method and apparatus disclosed in U.S. Pat. No. 6,081,093 is applied to a system in which a fluctuation of disturbance torque is great in an initial start period, such as that of a washing machine, the system may fail to initially start because of an instability operating caused by a large error in a real position of the rotor and a position command, and between the velocity and the velocity command.